The MHD jetting device of the present invention accomplishes the direct fabrication of metal parts by forming the desired part through the precise application of metal droplets upon a thermal sink. This device and method of use are suitable for rapid prototyping, free-form fabrication, and low volume manufacture of metal parts. Direct manufacture in metal using on-demand droplet deposition, as presently described, contrasts with direct manufacture processes of the prior art such as stereolithography (SLA), selective laser sintering (SLS), and three dimensional printing (3DP). SLA involves the use of photocurable resins and a laser to produce parts by sequentially building cross-section layers. The shape of the layers is directed by a computer by controlling exposure of the resin to the laser. This general process is described in U.S. Pat. No. 4,929,402 to Hull. SLS involves the use of powder layers, which are sintered by laser to produce three-dimensional parts. This process is described by Deckard in U.S. Pat. No. 4,863,538. 3DP is used to create a solid object by ink-jet printing a binder into selected areas of sequentially deposited layers of powder. Any unbound powder remains to support unconnected portions of the component as the layers are created and joined, and is removed after completion of the printing. 3DP is described in U.S. Pat. No. 5,204,055 issued to Sachs. Sanders Modelmaker (MM) and the IBM Genisys (marketed by Stratasys) are variants of the 3DP process, and exhibit the same limitations, as discussed more fully below.
The above-described processes and commercial embodiments are well adapted for use with plastic materials, such as polymers, but have significant drawbacks when done using metal. First, because the processes involve the use of powdered starting product, there is a need to manufacture, store and handle metal powders. This can be a particularly nettlesome process, fraught with safety concerns and with a need to protect reactive metals from the atmosphere. Second, the need for consolidation of the manufactured layers often requires the part to be subjected to high temperatures that may cause undesirable phase transitions in the metal. As described below, this problem is avoided by other manufacturing processes, such as ballistic particle manufacturing (BPM), which used cold welding of the particles on impact. Finally, there is no convenient means for arbitrarily adjusting the composition of the part in different locations. It is difficult to deposit the correct proportion of powder in the right place and to keep it there without disturbance until the part is consolidated. For these reasons, the present invention offers significant advantages over these direct manufacturing processes of the prior art.
Two other direct manufacturing processes of the prior art are ballistic particle manufacturing (BPM) and fusion deposition modeling (FDM). BPM utilizes an ink-jet apparatus to "print" successive cross-sections of the object to be formed. The layers are bonded using either cold welding or a rapid solidification technique. A corporation in Greenville, S.C.--BPM, Inc.--manufactures commercial BPM systems. Automated Dynamic Corp. of Troy, Mich., manufactures apparatus utilizing BPM with metals or metal composites. FDM involves the continuous extrusion of thermoplastics from an inkjet-like printhead. It has not been adapted for metals and lacks the control of on-demand ejection of individual metal droplets of desired composition, as presently disclosed. A FDM system is available from Stratasys, Inc. of Minneapolis, Minn. The present invention is advantageous when compared to these processes because it uses metal, relies on the natural bonding of molten droplets to the solid part being built, and can be easily adapted for production of arbitrary variations in composition, as described in detail below.
The present invention's drop-on-demand approach represents a considerable improvement over the continuous stream metal jetting devices of the prior art. These prior art devices broke a continuous stream of solder into uniform droplets using a piezoelectric crystal. Such a system is described by Hayes et al. in "Picoliter Solder Droplet Dispensing" Proceedings of the International Symposium on Microelectronics, ISHM '92 (October 1992) and Hayes and Wallace, "Solder Jet Printing for Low Cost Wafer Bumping," Proceedings of the International Symposium on Microelectronics, ISHM '96 (October 1996). After leaving the nozzle, each solder droplet passes through a charging electrode system where it acquires a uniform charge. The charged droplets can then be steered to specific positions on the target substrate using electrostatic plates. Devices of this type have been demonstrated that emit solder droplets as small as 25 .mu.m in diameter. However, droplet emission from a continuous stream jetting device cannot be rapidly turned on and off. Consequently, droplet deposition on the substrate must be interrupted by diverting the solder stream into a catch reservoir, resulting in significant waste. Furthermore, piezoelectric crystals suffer depolarization at elevated temperatures resulting in decreased performance. To date, piezoelectric crystals have not been used in jetting devices operating at temperatures above those require for molten eutectic SnPb solder (around 200.degree. C.).
Therefore, there is a need in the art for a liquid metal jetting technique which provides the deposition of nanoliter size droplets of molten metal with high positional accuracy using methods analogous to those developed by the inkjet printing industry. The use of a MHD drive makes it unique in its ability to translate electrical signals directly into tailored pressure pulses to dispense high temperature metal droplets. These electrical signals are derived from computer-aided design (CAD) data, resulting in precise repeatable droplet deposition with high flexibility. However, the formation of single uniform molten metal droplets from a jetting orifice is a complex process dependent on many factors including the shape and size of the jetting orifice, the surface tension and viscosity of the metal, the wetting interaction between the metal and the orifice material, and the configuration and timing of the electrical signal/pressure pulse applied to the molten metal. The present invention describes a metal jetting device and a method of use of that device which has maximized the variables listed above for the direct formation of metal parts.